An orderly development of paranodal axoglial junctions and bracelets of Nageotte in the rat sural nerve.

The present study was designed to assess the normal development of the paranodal apparatus with particular emphasis on axoglial junctions (AGJs) which constitute the paranodal barrier system. The sural nerve was examined in 10- and 31-day-old rats. During the early phase of myelination AGJ attachment of terminal myelin loops to the axolemma proceeded from the node to the internode. The frequency of terminal loops with AGJ attachment increased with fiber growth. As myelination advanced internodal-most loops became almost 100% attached to the axolemma by AGJs, whereas at the same time an increasing number of nodal-most loops were unattached, suggesting a lack of AGJ formation at this site. The formation of bracelets of Nageotte increased with the progressive addition of myelin loops. They formed most frequently at the juxtanodal interface between unattached and attached loops, probably reflecting crowding of terminal loops along the unchanged length of the paranodal axolemma. The findings suggest a complex but orderly age- and fiber size-dependent maturation process of the paranode and its structural barrier system. The present data will serve as a basis for the evaluation of this anatomical region in regenerating and remyelinating fibers in various neuropathies.